MICROSCOPIC STUDIES OF STATIC AND DYNAMIC PROPERTIES IN QUANTUM LIQUIDS AND GASES

In this thesis I present studies of a number of quantum many-body Bose systems via Quantum Monte Carlo methods. We investigated the dynamic structure factor of a hard-sphere Bose system simulated at T=0 at different densities, from the dilute to the strongly interacting regimes. By increasing the density we observed the spectrum evolves from the weakly interacting Bogoliubov to a phonon-maxon-roton dispersion, but also the emergence of a broad multi-quasiparticle component. For a system with sphere radius and density corresponding to superfluid 4He at equilibrium, we found good agreement with the spectrum in the roton region. In another work, a liquid of distinguishable 4He atoms near freezing at T=1 K was studied to compute the equation of state and static density response function. The results of this study have been used to improve the description of the superfluid-to-solid transition within the Density Functional Theory. Measurements of crystallization kinetics in supercooled liquid p-H2--o-D2 mixtures showed a slowdown with respect to the pure counterparts. In order to contribute to the interpretation of these results we simulated these metastable mixtures. We found differences in the quantum delocalization of the two isotopic molecules, which result in different effective sizes. We characterized also the differences in the local order around the molecules of each species. These results revealed that the observed slowdown is due to purely quantum effects. Finally, in a QMC study of ion Ar+ doped 4He nanodroplets at T=1 K, we computed density profiles, energies, and investigated local order around the Ar+ ion. We found stable solid structures around the ion composed of three solvation shells having the shape of platonic solids: an icosahedron, a dodecahedron, and, again, an icosahedron, with 12, 20, and 12 4He atoms, going from the inner to the outer shell respectively. These results confirmed the interpretation of experimental measurements of the abundances of Ar+@4He nanodroplets

Modelling of Silicon-Germanium Alloy Heterostructures using Double Group Formulation of k . p theory

Silicon-Germanium alloy heterostructures offer the most viable opportunity to integrate electronics with optoelectronic devices for widespread commercial application. Indeed Germanium rich devices may be designed for application around 1.5 m by preying on the direct-gap energy of 890meV. Low power optical modulators operating, under the quantum confined Stark effect, at wavelength bands used in 3rd generation fibre optic communication channels are developed in this thesis from a theoretical perspective. An investigation into strained Germanium rich quantum well structures was performed, revealing information about sub-band dispersion, joint density of states and absorption coefficient using the double group formulation of k . p theory. Using zone centre eigenstates as symmetrised half integer basis functions transforming according to irreps of the double group, the spin orbit interaction is incorporated into the unperturbed Hamiltonian. Along with semi-empirical input parameters available in the literature, dispersion in bulk Silicon and Germanium reveals information about hole effective masses and indirect conduction band minima in broad agreement with experimental data. In accordance with degenerate perturbation theory; effective mass Hamiltonians, with an arbitrary quantisation axis through a canonical transformation, are constructed through a series of matrix multiplications. Retaining operator ordering allows numerical modelling of heterostructures grown on arbitrary growth planes with appropriate boundary conditions across an abrupt interface under the envelope function framework. In this thesis, the effect on the transition energy, hh1-e1, by the choice of growth plane in a quantum well heterostructure is investigated

Software for Exascale Computing - SPPEXA 2016-2019

This open access book summarizes the research done and results obtained in the second funding phase of the Priority Program 1648 "Software for Exascale Computing" (SPPEXA) of the German Research Foundation (DFG) presented at the SPPEXA Symposium in Dresden during October 21-23, 2019. In that respect, it both represents a continuation of Vol. 113 in Springer’s series Lecture Notes in Computational Science and Engineering, the corresponding report of SPPEXA’s first funding phase, and provides an overview of SPPEXA’s contributions towards exascale computing in today's sumpercomputer technology. The individual chapters address one or more of the research directions (1) computational algorithms, (2) system software, (3) application software, (4) data management and exploration, (5) programming, and (6) software tools. The book has an interdisciplinary appeal: scholars from computational sub-fields in computer science, mathematics, physics, or engineering will find it of particular interest